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How did scientists define mass and force back in the days?

  1. May 30, 2015 #1
    Hi, I'm amazed by all the tools we have in science to measure things indirectly fx measure weight(force) by measuring mass and downward acceleration of an object.

    But there seems to be some hidden secrets: How do you invent a concept like mass and force and measure them in the first place? We know that you need force and acceleration to measure mass, but to make equipement like a newton meter to measure force, you need a known mass to calibrate it to measure force accurately. which invention came first, and how could you define the size of one of them without knowing the other?
  2. jcsd
  3. May 30, 2015 #2
    I think Mr. Anderson just explained it here: Someone probably just used the distance formula m1*d1=m2*d2 and found an object and said "I will assign this object a mass of 1kg" and then used other objects to solve for their mass. So mass is a relationship between 2 mases and their distances from a center point on a balance according to Gravitational mass method. This way of determining mass must have come before defining intertial mass, because you need mass to calibrate a tool that can measure force.

  4. May 30, 2015 #3


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    You seem to be treating things like newtons and kilograms as some sort of absolute units. They are not. All measurement units are established by some sort of convention, where a group of people get together and say, a meter is this long, or a kilogram is this much of a substance.



    Because units are based on a convention, the convention can be altered over time, and the definition of various units can subtly change. For instance, a meter is no longer the length of a certain metal rod kept in a vault in Paris, but is now defined as so many wavelengths of the orange-red emission line of Krypton-86.

  5. May 30, 2015 #4


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    The kilogram is the last unit that still relies on an object (a cylinder in Paris that has a mass of exactly 1 kilogram by definition). Multiple groups try to replace this by a more reliable definition in terms of fundamental physical constants, that would allow to gauge scales without this object in Paris.
    Historically, most units were defined based on objects - like a stick that was one meter long by definition, and so on.
  6. May 30, 2015 #5
    I am aware that every unit we made up is "made up" and often relies on other made up units to measuer it indirectly. but I'm interested in the ways people would measure these things back in the days, also how the gravitational constant, electrical constants etc. was measured back then, and then how our modern equipement measures theese things today, but i don't really know if there is any book that goes into detail about that? I'm very interested in the cronological order, so how the first units were made up, then how they were used to derive the next unit (fx the Gravitational constant must have relied on other units and methods to calculate them in order to derive the gravitational constant). I just don't know where to find this information.

    I was wondering if what i'm searching for has a name? i stried searching on amazon for "History of SI units" or "development os measurement techniques" but i only find charts.
    Last edited: May 30, 2015
  7. May 30, 2015 #6


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    Searching on Amazon is not the best way to find out details about a scientific subject. After all, Amazon is a merchant retailer of books, electronics, appliances, etc.

    If you want to know how various constants were first measured, stick to web search engines, if you don't have access to anything better.

    You must realize that things like the gravitational constant and the various electrical constants couldn't be measured until after people had developed a notion of what gravity is, or what electricity is.

    For instance, if you do a web search on "gravitational constant", you will not only find an accurate value of this in modern units, but also articles which deal with its history and how it is measured:


    Similar results can be obtained for other physical constants. To a great extent, it depends on where you look and how long you are willing to look to find this information.

    Amazon would be a handy resource if you were looking for a book on the history of science, or the history of physics, for example, which reading would give you a broad introduction to such topics as have been discussed in this thread.
  8. Jun 9, 2015 #7


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    Which is kind of funny, because originally it was defined just as the mass of one litre of water at a specified temperature.
  9. Jun 11, 2015 #8
    In classical mechanics the concept of mass and force have been invented with Newton's definition 2 (classical momentum), lex 2, lex 3 and isotropy.

    As already explained, by comparison with a reference. This reference can simply be based on a definition (e.g. 1 l of water at maximum density has a mass of 1 kg) or it can be derived from other references (e.g. 1 N is the force needed to accelerate 1 kg with 1 m/s²).

    To measure mass you need to keep force and acceleration equal for the unknown mass and the reference mass. But you don't need to measure them.
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